验证多边形是否为凸多边形
#include <stdio.h>
#define _DEBUG 0
int crossMulti(int x0,int y0,int x1,int y1,int x2,int y2){//差积
return (x1-x0)*(y2-y0)-(x2-x0)*(y1-y0);
}
int main(){
int n,i;
int f_x,f_y;//第1个点
int s_x,s_y;//第2个点
int p0_x,p0_y;//上2个点
int p1_x,p1_y;//上一个点
int cur_x,cur_y;//当前点
double area = 0;
#if _DEBUG == 12108 Shape of HDU
freopen("2108.in","r",stdin);
#endif
while(scanf("%d",&n) && n != 0){
scanf("%d %d",&f_x,&f_y);
scanf("%d %d",&s_x,&s_y);
p0_x = f_x;p0_y = f_y;
p1_x = s_x;p1_y = s_y;
bool flag = true;
for(i=2;i<n;++i){
scanf("%d %d",&cur_x,&cur_y);
if(crossMulti(p0_x,p0_y,p1_x,p1_y,cur_x,cur_y) < 0){
flag = false;
if(i != n-1){//取完本行数据,注意
char tmp[100];
gets(tmp);
}
break;
}
p0_x = p1_x; p0_y = p1_y;
p1_x = cur_x; p1_y = cur_y;
}
if(crossMulti(p0_x,p0_y,p1_x,p1_y,f_x,f_y) < 0
|| crossMulti(p1_x,p1_y,f_x,f_y,s_x,s_y) <0){//第1、2个点的,特别要注意
flag = false;
}
if(flag){
printf("convex\n");
}else
printf("concave\n");
}
return 0;
}